The present invention relates to a basic refractory composition that is particularly suitable for use in kilns and furnaces.
Magnesite-chrome, magnesia, magnesia-spinel, calcia, dolomite and magnesia-carbon have been used as conventional basic refractory compositons. These compositions are chiefly produced from natural and seawater magnesia clinkers, natural dolomite clinkers, synthetic magnesia-dolomite clinkers, calcia clinkers, chromite, spinel clinkers, natural graphite, etc. These materials, used either singularly or in admixture, are used as castable refractories or shaped refractories that may or may not be fired.
Various basic refractory compositions are used in sintering furnaces (e.g. rotary kilns), steelmaking ladles, secondary refining furnaces, converters and other melting furnaces depending upon the specific properties to be required of the refractory compositions, such as resistance to thermal spalling, resistance to erosion by molten metal, slag and cement clinkers, resistance to reaction with slag and cement clinkers and resistance to stresses such as load in high temperature.
In the firing zone of a rotary cement kiln, fired bricks are used, mostly in the form of magnesite-chrome bricks and magnesia spinel bricks and occasionally in the form of dolomite bricks. The first two types of fired bricks are designed to have quality that meets such requirements as resistance to erosion by cement materials, resistance to thermal spalling and the stability of coating adhering. Calcia bricks are unsuitable for use in the firing zone of a rotary cement kiln since they have a great tendency to be slaked by moisture absorption. Magnesia bricks are also unsuitable because of their poor resistance to thermal spalling. Magnesia carbon bricks are unuseful, too, since carbon is oxidized during extended use. Dolomite bricks have the advantage of consistency in coating adhering but, on the other hand, they are poor in resistance to thermal shock. Even commonly used magnesite-chrome bricks suffer from the disadvantage that the iron oxides present in chromite change from the divalent form to the trivalent form, or vice versa, during use, and that this reduces gradually the strength of the brick by destroying its structure. Magnesia-spinel bricks also have the defect in that the spinel clinker in the brick is easily decomposed as a result of reaction with raw cement materials, introducing instability in the adherence of cement coating. The prior art refractories used in rotary cement kilns have the aforementioned problems and there exists a need to further lengthen their service life.